This invention relates generally to an electrophotographic printing machine, and more particularly concerns an improved printing process for producing high quality color reproductions.
In the process of electrophotographic printing, a light image of an original document exposes a charged photoconductive surface. The irradiated area of the photoconductive surface are discharged recording thereon an electrostatic latent image corresponding to the original document. A development system positions a developer mix of carrier granules and toner particles in contact with the electrostatic latent image recorded on the photoconductive surface. Toner particles are attracted electrostatically from the carrier granules to the latent image forming a toner powder image thereon. The toner powder image is then transferred to a sheet of support material. After the toner powder image is placed on the sheet of support material, it advances to a fuser which permanently affixes the toner powder image thereto. This process is more fully described in U.S. Pat. No. 2,297,691 issued to Carlson in 1942.
With the development of magnetic brush systems and electrically biased electrodes for cascade development systems, it has been feasible to reproduce images containing lines and solid areas of various shades. The introduction of screening techniques have further extended the range of electrophotographic printing to half-tone images. However, it is extremely difficult to create the full latitude of tone contrast that is possible in conventional photography. Techniques which accomplish this require extremely complex equipment and highly sensitive controls which must be continually monitored during the printing process.
The technique of increasing the cynamic range or the brightness acceptance of an electrophotographic printing machine is frequently referred to as extended range electrophotographic printing. One approach to extended range electrophotographic printing is described in U.S. Pat. No. 3,212,889 issued to Gundlach et al in 1965. As described therein, a photoconductive plate is charged, exposed to a light image of a pre-selected magnitude, developed and transferred to a support sheet. Residual toner particles are cleaned from the photoconductive plate and the photoconductive plate is then resensitized and re-exposed to a second light image or the original document having a different magnitude than the first light image thereof. The second image is developed and transferred, is superimposed registration with the first image, onto the common sheet of support material. The foregoing process may be repeated a plurality of times to form successive toner powder images for different magnitudes of exposure. Another approach is described in co-pending application Ser. No. 497,547 filed in 1974. As described therein, different concentrations of toner particles within the developer mix are employed to develop the electrostatic latent images recorded on the photoconductive surface. The foregoing electrostatic latent images are also produced with different magnitude light images.
It is a primary object of the present invention to improve electrophotographic printing so as to form half-tone copies having substantially the density of the original document.